The user in the database will be an orphaned account from the old server, is it a sql or domain account?.
If you check in the server logins rather than the database logins it is likely that this account doesn't exist unless you have added it to the new server. Try removing it from the database, then add it to the server logins if not already added then add it back to the database again.

Thank you for your reply - it is an sql account. Yes, orphaned sounds right, but when I delete the server login from the database and re-create it, it will not let me "user mapping" it, because of the attached error above.

When I also tried to delete the orphaned database account from the database users, it says the account is being referenced by a schema - and that the database principal owns a schema in the database?

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